The invention relates to a circuit arrangement for operating electrical lamps. This type of circuit arrangement is used, in particular, for operating low-voltage incandescent halogen lamps, on the one hand, and discharge lamps, for example fluorescent lamps, on the other hand, from an AC voltage power supply or a DC voltage source. Circuit arrangements of this type are generally called "electronic ballasts" (EBs) for the operation of discharge lamps, while the designation "electronic transformer" or "electronic converter" is customary for the operation of low-voltage incandescent halogen lamps.
An essential component is a self-excited invertor with current feedback, for example in a half-bridge or full-bridge circuit, which invertor chops the low-frequency power supply voltage or the DC voltage--which may also be pulsating--into a high-frequency voltage. The latter is adapted to the electrical requirements of the lamp(s) to be operated by means of a coupling circuit tuned to the type of lamp.
In order to operate low-voltage incandescent halogen lamps, the coupling circuit essentially comprises a power transformer which transforms the high-frequency voltage to the low voltage of the incandescent halogen lamps to be used, e.g. 6, 12 or 24 V. Therefore, such circuit arrangements are also referred to as electronic converters. Their detailed method of operation is disclosed for example in EP-V 264 765 and DE-A 40 11 742.
In order to operate discharge lamps, for example fluorescent lamps, the bridge transistors are followed by a ballast inductor which limits the lamp current. In order to ignite the discharge, the coupling circuit may also comprise a capacitor which is connected in parallel with the electrodes of the discharge lamp and is operated in resonance with the ballast inductor. The method of operation of such electronic ballasts is explained in more detail for example in DE-C 29 41 822 and DE-A 38 05 510.
In both cases, the invertor is controlled by an output current component that is fed back. As a result, in order to start the oscillation of the invertor for the first time (see e.g. EP 0 682 464 A1), for example immediately after the switching on of the supply voltage, and also in order to restart the oscillation after each zero crossing of the power supply voltage (see e.g. EP 0 682 465 A1 or EP 0 647 084 A1), in particular in electronic converters, a control pulse is necessary to initiate the HF oscillation of the invertor. Electronic converters or ballasts usually contain an oscillation build-up circuit, also called a start or trigger generator, which performs this task.
In the simplest case, the trigger generator essentially comprises a charging capacitor, a charging resistor and a voltage-dependent switching element, for example a diac. The charging capacitor is initially charged via a charging resistor. When the voltage of the charging capacitor reaches the threshold value of the voltage-dependent switching element--the triggering voltage of the diac in the example--the switching element turns on and the high-frequency oscillation of the invertor starts. During operation of the invertor, on the other hand, it is necessary to prevent the generation of a trigger signal that interferes with the high-frequency oscillation through the voltage-dependent switching element.